1. Field of the Invention
The present invention relates to an apparatus for adsorbing and concentrating gases, such as volatile organic compounds (referred to hereinafter as "VOCs") and other gases, from air.
2. Description of the Related Art
Pressurized containers filled with adsorbents such as active carbon and zeolite have long been known in the art for the concentration and removal of VOCs and others gases contained in air. They function by passing air containing VOCs through the containers to adsorb the VOCs and other gases on the adsorbent. Next, the pressurized container is decompressed and the air containing the adsorbed VOC is discharged to concentrate the VOC. That is, adsorption and desorption are done through pressure changes. However, such devices are large and very expensive.
Therefore, an adsorption rotor was fabricated in a honeycomb shape from adsorbent carrying sheets. One example of adsorbent carrying sheets is described in International Patent Application No. WO91/16971. Air containing a VOC is passed through the adsorbing zone of a rotating adsorption rotor to adsorb the VOC. Next, a hot wind is sent through the desorbing zone, desorbing the VOC adsorbed on the rotor and concentrating the VOC.
This honeycomb-type concentrating and removing apparatus is smaller and less expensive than a pressure swing-type concentrating and removing apparatus. However, the concentration ratio of the honeycomb-type concentrating and removing apparatus is limited to about ten (10) times.
Recently, there is an increasing demand to remove VOCs from air containing low VOC concentrations. For example, liquid crystal display manufacturing plants produce gas having a low VOC concentration. The honeycomb-type concentrating and removing apparatus often fails at removing low VOC concentrations, however, discharging the air without treating the VOC. Nevertheless, even if the concentration of the VOC contained in the air is low, the need to recover the VOC is high.
Further, self combustion cannot be maintained if the air containing VOC has a low concentration. For example, VOC contained in the air at a concentration of about 50 ppm can be concentrated to about 500 ppm by using the honeycomb-type concentrating and removing apparatus. Unfortunately, self-combustion of the VOC contained in the air at 500 ppm concentration cannot be maintained. Therefore, methane or other gases are mixed with the VOC-containing air as back-up fuels to increase the concentration of the burnable gas.
In many cases, methane or other gases are mixed with the VOC containing air to get the burnable gas concentration to about 1000 ppm for treatment by a combusting device. In other words, the gas output from a concentrating device is mixed with methane (usually natural gas) to achieve a gas that can be burned.
When the concentration of gas increases, the quantity of the added methane gas is decreased, and when the concentration of the gas decreases, the quantity of the added methane gas is increased. And when the quantity of the added gas is increased, the emission of carbon dioxide from combusting device also increases.